We were not able to reproduce the reported issue at our end and we have prepared a sample for this. Please update us the modified sample to reproduce the issue which will be helpful for providing further assistance on this.

Regarding “but I am unsure how as there are no permissions set in the manifest - particularly for accessing files?”

You no need to enable Read/Write permission to save Chart as an image. In source level, we save this chart as an image by creating a new file (name of the image file) under Pictures folder of ExternalStorage.

Are you able to run our sample in the device in which you are facing this issue? If yes, can you able to see the chart image under Pictures folder?

If not, please update us the modified sample or update us the with more details (Device details) in which you are facing this issue which will be helpful to assist you further.